Ok so I am going to get my replacement body tomorrow (flew in too late to pick up on saturday). However I have just come back from a pre-wedding shoot this afternoon and the same flash related issue popped up again.

Equipment list:

1DX600EX RTEF 70-200 F2.8 L IS USM IISandisk 60MB/s 8GB CF and Sandisk 90MB/s 32GB CF (i normally use match pairs but didn't have time to clear the full card prior to the shoot)

Now in single shot mode it works fine. But i had the couple move around a bit so switched to AI Servo and this is where the issue begins.

The camera literally locks up - screens are powered, vf is powered but the lens doesn't rack in portrait or landscape, depressing either shutter does nothing, powering off the body does nothing.

Turning the flash off and turning the body off fixes the issue for a few frames but after a good 20 frames it repeated the issue.

-Switching lens to a 50mm and the issue didn't come up again.-When this first happened overseas I was using the same configuration 600EX with 70-200.-On a 7D this same config doesn't have the same issue.

Ok, so - replicated indoor with room lights, not the best test as the servo has issues in this room focusing normally. Remembering issue occurs during midday light on my unit.

-When it locks up, flash fires from test button.-If i continue to mash the shutter over and over after a good 15-20 presses the lens suddenly springs to life.-It did however take 23 shots before the issue occurred - i started at -3fec and ramped up to -1fec (not that it really matters)